FIG. 1 illustrates a configuration of an optical network of related art. As illustrated in FIG. 1, control nodes (CNT) 2a through 2f are arranged on a control plane respectively for optical nodes 1a through 1f, including optical cross-connect (OXC) on a data plane.
An optical path may be established from the optical node 1a to the optical node 1f. Each control node stores detailed physical information about a link between an optical node corresponding to a source control node and an adjacent optical node. The detailed physical information may include, for each of a plurality of modulation methods, SNR (signal to noise ratio) degradations, chromatic dispersion, polarization mode dispersion (PMD), and non-linear effects such as Kerr effect of optical fiber. The control node 2a corresponding to the optical node 1a corrects the detailed physical information of links of control nodes 2b through 2f with optical nodes 1b through 1f. The control node 2a reserves and establishes an optical path (1a, 1b, 1c, 1e, and 1f) connecting the optical node 1a down to the optical path 1f denoted by a heavy solid line with respect to each selectable modulation method in accordance with parameters. The parameters include baud rate, forward error correction (FEC) overhead rate, optical signal noise ratio (OSNR) tolerance, chromatic dispersion tolerance, PMD tolerance, and non-linear effect tolerance.
As a related technique, a transmitter intensity-modulates (amplitude-modulates) an optical data signal with a low-speed path ID tone signal and then transmits the modulated optical data signal to a receiver. The receiver demodulates the path ID tone signal, thereby determining a modulation method and baud rate to establish a path. Related techniques are also described in U.S. Pat. No. 7,580,632.
As a related technique, a bias of an electroabsorption type modulator (EA modulator) at a transmitter is controlled in accordance with error rate information detected by a receiver so that chirp of an optical modulation signal is controlled to an optimum point. Transmission of a control signal from the receiver to the transmitter may be performed by digital multiplexing the control signal on an optical main signal. Related techniques are also described in Japanese Laid-open Patent Publication No. 2002-164846.
As a related technique, a receiver measures transmission quality with a transmission characteristic measurement unit thereof, and transmission parameters are controlled to optimum values in response to measurement results. The transmission parameters include optical transmission power, optical wavelength, dispersion compensation amount, and pre-chirp. Related techniques are also described in Japanese Laid-open Patent Publication No. 2005-223944.
As a related technique, a receiver measures OSNR, and bit error rate (BER) of each wavelength of a wavelength-division multiplexed (WDM) signal, and transmits measurement results in overhead information of an optical signal to a transmitter. The transmitter optimum-controls a parameter, dispersion compensation amount, and pre-emphasis of an optical transmission signal. Related techniques are also described in Japanese Laid-open Patent Publication No. 2002-57624.
Japanese Laid-open Patent Publication No. 8-298486 discloses a superimposed transmission technique in which an optical main signal is low-intensity-modulated with a supervisory control signal.
A plurality of modulation methods are characterized by a variety of parameters including baud rate, FEC overhead rate, OSNR tolerance, chromatic dispersion tolerance, PMD tolerance, and non-linear effect tolerance. Optical path establishing methods of related art perform a number of steps before establishing an optical path optimum for highly efficient data transmission. Design for establishing the optimum optical path is difficult.
In the related technique where the optical data signal that is optical-intensity modulated with a low-rate signal is transmitted and the receiver demodulates the low-rate signal, cross-gain modulation caused through an optical amplifier and stimulated Raman scattering causes a path ID tone signal from another channel to enter. As a result, quality degradation takes place on a main signal.